It is well known that the useful light output of a discharge lamp will diminish progressively in relation to the number of hours that the lamp has been in operation, i.e. burning time, this relationship being sometimes referred to as the lumen maintenance. It is evident from tests carried out in this regard that lamp luminescence is affected negatively as a result of interactions that take place between the substances incorporated in the glass surface of the discharge tube, the luminescent powder granules, the mercury, the electrode devices, and gaseous contaminants in the lamp atmosphere, e.g. gaseous nitrogen, carbon monoxide gas, carbon dioxide, water, and hydrocarbons. These interactions may possibly result in chemical reactions which:
reduce the physical efficiency of the luminescent powder; PA1 lessen the ability of the discharge tube to allow visible light to pass through; and PA1 contaminate the lamp atmosphere and thereby negatively influence the light characteristics of the lamp and its length of useful light.
A discharge lamp, such as a fluorescent lamp, in which the discharge tube or glass envelope is made of soda glass and contains mercury, fluorescent powder, emission substance, a noble-gas fill and diverse gaseous contaminants, constitutes a highly reactive system, from a chemical viewpoint. The chemical reactants engendered through the discharge mechanism thus take part in many different reaction processes and often influence the speed and the state of equilibrium of these processes. The transition to discharge tubes of smaller diameter that has taken place in recent years has resulted in an increase in the wall load of such tubes per unit of glass area. In this way the proportion of energy-rich ultraviolet light generated (wavelength=185 nonometers, nm) has increased in relation to the excitation radiation (wavelength=254 nm). This higher density of UV-185 nm--which is a consequence of the high electron temperature in the plasma formed by the positive column extending between the anode and cathode of the discharge tube--results in greater occurrence of energy-rich chemical reactants.
Consequently, in the manufacture of discharge tubes, or glass envelopes, of progressively decreasing diameters progressively higher demands are placed on lamp construction with regard to the choice of those components which can be expected to be subjected to chemical attack as a result of the comparatively high UV-intensity. In the case of lamps such as these, inter alia, the mercury atoms--when excited to a level of 6.sup.1 P.sub.1 (6.7 eV)--constitute a threat to certain luminenscent powders when the crystal lattice, or activator centres, of such powders incorporate cations which have an electronegativity greater than 1.5 Pauling units. As is well known, excited mercury atoms will react with oxygen atoms already at room temperature, in accordance with the formula: ##STR1##
The reaction product is thus mercury oxide which condenses in the form of a colored light-absorbent coating on various parts of the lamp discharge chamber, primarily on the layer of luminescent powder present in the area around the Faraday dark space, where the presence of positive and negative charge carriers is relatively high (inter alia Hg.sup.+, Hg.sub.2.sup.30, O.sup.--).
Another factor which is liable to lower the luminescence of a lamp is the interaction of the UV-radiation and the reaction of the Hg-atoms with the substances incorporated in the glass surface of the discharge tube. It is well known that different compositions of glass have different degrees of sensitivity to radiation which is rich in energy. This phenomenon is called solarization and is the effect of photochemically initiated redox reactions (electron transfers) between the types of atoms present in the glass, often metal ions in an oxide. These photochemical processes often lead to a change in color of the glass (discoloration) with a subsequent reduction in permeability to visible light. The interaction of UV-radiation with glass can, in some cases, also cause mercury atoms to become involved secondarily in a process which produces other light absorbing compounds, such as HgS for instance.
During the years in which the development of mercury discharge lamps has progressed, a number of papers have been published. The problems associated with the reaction of mercury with amalgam-forming atoms incorporated in the glass discharge tube and with the light absorbing properties of the resultant reaction products have been discussed in these papers.
The extent to which amalgam is formed during the various stages of the useful life of the lamp depends greatly on the composition of the glass from which the lamp envelope is made and on the condition of the surface of the glass. The glass surface may be activated with amalgam-forming reactants by diffusion of alkali from internal parts of the glass even in the manufacturing stage of the lamp, when the binder present in the luminescent powder layer is baked off in a furnace at ca 600.degree. C. The formation of alkali amalgams which are colored to greater or lesser degrees, e.g. Na.sub.n Hg.sub.m (n,m=1-8), is considered to be one of the reasons for abnormal light losses.
Since the risk of photochemical reactions (solarization) and of the formation of amalgam becomes greater with decreasing tube diameters (higher wall loading per unit of area), it is desirous to hold the reactants separated to the greatest possible extent. Those luminescent powders which are adapted for optimum light emission, e.g. luminescents of the 3-band kind, will not normally fulfill the requirements placed on an effective barrier against such amalgam formation. The most suitable particle size distribution for maximum light generation is such (2-8 micro-meters) that the powder layer is relatively porous and therewith forms a poor mechanical barrier against mercury vapour. In addition hereto UV-185 nm is reflected badly by the relatively coarse particles of luminescent powder and will pass essentially therethrough, since absorption of this energy-rich radiation is low. Consequently, approximately 50% of the UV-185-radiation reaches the glass surface of the lamp envelope, where it is able to initiate various chemical or photochemical processes.